The drug carbamazepine (CBZ) is widely prescribed for the treatment of various conditions including attention-deficit disorder, bipolar disorder, schizophrenia, and epilepsy. In patients, CBZ has a reported half-life of 18-65 hrs and is metabolised mainly to carbamazepine-10,11-epoxide (CBZ-E); 10,11-dihydro-10-hydroxycarbazepine (10-OH-CBZ); 10,11-dihydro-10,1-dihydroxycarbazepine (10,11-diOH-CBZ); and to minor amounts of the corresponding glucuronides. The major urinary excretory product is 10,11-diOH-CBZ (80%). The therapeutic range is reported as 2-10 μg/ml for total carbamazepine i.e. free and protein bound, and 0.5-3.6 μg/ml for free carbamazepine, the corresponding toxic values given as greater than 12.00 μg/ml and 4.00 μg/ml, respectively (Mayo Clinic, Mayo Medical Laboratories, 2011). Toxic concentrations can provoke drowsiness, headaches, affect motor function and in extreme cases are fatal. This narrow therapeutic index allied to inter-patient variability in drug-responsiveness requires that the blood concentration of CBZ and its main active metabolite CBZ-E be monitored in patients on CBZ therapy. The immunoassay is a common analytical test format used for the therapeutic drug monitoring (TDM) of patients on drugs such as CBZ and are based on antibodies that bind the target analyte(s). Several immunoassays/immunoassay components are described that bind carbamazepine and one or more metabolites at various concentrations e.g. EP 1216994, U.S. Pat. No. 5,851,779, EP0584854, EP0583820, U.S. Pat. No. 6,803,040. Several commercial immunoassay kits of varying specificity are available, many of which describe a TDM use for patients on carbamazepine therapy (Table 1). TDM applications based on immunoassays are best suited by antibodies that have high specificity for either the parent drug or an active metabolite to enable accurate measurements of its concentration, whereas toxicological applications do not necessarily have this specificity requirement.
Oxcarbazepine is a recent anti-epileptic drug that rapidly metabolises to 10-OH-CBZ and is considered a candidate for TDM (Bring and Ensom 2008; Krasowski 2010). An antibody that binds to oxcarbazepine/10-OH-CBZ could be used in an immunoassay for monitoring in vitro samples of patients undergoing oxcarbazepine drug therapy. Such an antibody could also be applied in an immunoassay for the TDM of other drugs of the carbamazepine family that produce 10-OH-CBZ as a metabolite. Immunoassays incorporating such antibodies could also have other applications, such as in toxicology. None of the immunoassays described in the prior art (including commercial assays described in Table I) have substantial cross-reactivity towards oxcarbazepine/10-OH-CBZ and are therefore unsuitable for such purposes. The cross-reactivity profiles of the commercial assays displayed in Table I suggest immunogens used to raise the antibodies of the assays were derived through derivatisation at the carboxamide/l-position of a carbamazepine/carbamazepine-like hapten (pre-immunogenic, small molecule). The extent of consumption of pharmaceuticals has resulted in concerns over the levels of drugs and their metabolites in the aquatic environment, one of which is carbamazepine. Detection of pharmaceutical drugs such as carbamazepine in drinking water and environmental water bodies for their ecotoxicogical effects is desirable. A recently described CBZ immunoassay incorporating a monoclonal antibody detected CBZ and CBZ-E but had negligible cross-reactivity towards oxcarbazepine, 10-OH-CBZ and 10,11-di-OH-CBZ (Bahlmann et al. 2009). It is probable the amount of carbamazepine in water bodies was underestimated by this immunoassay as the antibody showed negligible binding to the major urinary metabolite of carbamazepine, 10,11-di-OH-CBZ. Antibodies of current carbamazepine immunoassays do not cross-react with 10,11-di-OH-CBZ and would therefore underestimate levels of carbamazepine drug in the aquatic environment. A preferred immunoassay would find application in the TDM of carbamazepine, of newer carbamazepine-based drugs and for the monitoring of carbamazepine-based drugs in the environment. It is evident that current immunoassays, in which the assay antibodies have high affinity for CBZ and CBZ-E, do not possess this broad applicability.